MEMS microphones usually comprise a conductive backplate and a conductive, flexible membrane being arranged at a distance from the backplate. The backplate and the membrane realize electrodes of a capacitor. When a bias voltage is applied to the electrodes, oscillations of the membrane caused by received acoustic signals are converted into electrical signals. For further signal processing MEMS-microphones can comprise an ASIC (Application-specific Integrated Circuit) chip.
Such microphones have a central, acoustically active area and a suspension area. The central area is surrounded by the suspension area that is acoustically inactive. Within the suspension area, suspension means for mechanically and/or electrically connecting the backplate and/or the membrane are arranged on a substrate material. For constructing MEMS microphones, manufacturing processes of semiconductor devices such as layer deposition, deposition of photo resist films, structuring photoresist films, and partly removing structured layers can be utilized. As a consequence of the necessity for suspension means the capacitor—apart from the central area—has an acoustically inactive area deteriorating the signal quality of the microphone. This deterioration is due to parasitic capacitance. The corresponding signal attenuation Hc is:Hc=Cm/(Cm+Ci+Cp),  (1)where Ci is the input capacitance of an according ASIC chip processing the electrical signal. Cp is the parasitic capacitance, i.e., the capacitance of the capacitor's acoustically inactive area. Cm is the total capacitance comprising the capacitance of the central area and the capacitance of the suspension area. Reducing the parasitic capacitance reduces the signal's deterioration and, thus, improves the microphone's signal quality.
Cp mainly depends on the acoustically inactive suspension area of the backplate overlapping a suspension area of the membrane.